Perceptual masking is an increased difficulty in understanding speech beyond conventional masking that occurs when background sounds are complex and include competing speech. The amount of perceptual masking has been shown to fluctuate, but the roles of the major variables responsible have not been defined. The purpose of this research is to study these variables. A series of experiments are planned to discover (1) whether perceptual masking follows the same rules monaurally as it does binaurally, (2) whether it becomes still greater as the primary message must be disentangled from successive trains of competing speech beyond two, (3) how perceptual masking changes as the type of the primary message is modified, (4) whether the overall level of the entire acoustic complex modifies perceptual masking, (5) what are the relative influences on perceptual masking of acoustic, linguistic and semantic similarities between the primary message and its competition, and (6) how perceptual masking is affected by quality of reproduction when the entire sound field is heard via an electroacoustic system, such as a hearing aid.